Add test for String#to_r with leading plus sign #379

Job	Run time
RuboCop	32s
specs (ubuntu, 3.1.6)	1m 33s
specs (ubuntu, 3.2.6)	1m 28s
specs (ubuntu, 3.3.7)	1m 27s
specs (ubuntu, 3.4.1)	1m 35s
specs (macos, 3.1.6)	1m 26s
specs (macos, 3.2.6)	1m 6s
specs (macos, 3.3.7)	1m 11s
specs (macos, 3.4.1)	1m 41s
specs (windows, 3.1.6)	3m 41s
specs (windows, 3.2.6)	4m 57s
specs (windows, 3.3.7)	4m 8s
specs (windows, 3.4.1)	3m 45s
specs (ubuntu, 3.3.0, --enable-frozen-string-literal)	1m 27s
	29m 57s

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